The present invention relates to equalization and decoding apparatus for a frequency-selective digital transmission channel.
In digital transmission, a receiver can be considered as being a plurality of elementary functions in cascade, each function performing specific processing such as filtering, demodulation, equalizing, decoding, etc.
In general, the processing performed by any one elementary function makes use of only a portion of the information made available thereto, and as a result the overall performance of a receiver is suboptimal.
An object of the invention is to remedy that drawback.
Various authors have already proposed overcoming the intersymbol interference that a channel introduces by using various types of receiver based on equalizers or detectors relying on maximum likelihood, and sometimes referred to by the misnomer xe2x80x9cViterbi equalizersxe2x80x9d. The most significant contributions in this field are the following:
[1] A. Gersho, and T. L. Lim, xe2x80x9cAdaptive cancellation of intersymbol interference for data transmissionxe2x80x9d, The Bell System Technical Journal, Vol. 60, No. 11, pp. 1997-2021, November 1981.
[2] M. S. Mueller, and J. Salz, xe2x80x9cA unified theory of data-aided equalizationxe2x80x9d, The Bell System Technical Journal, Vol. 60, No. 9, pp. 2023-2038, November 1981.
In those articles, the authors propose a receiver which combines an adapted filter and an interference canceller to eliminate interference between symbols as introduced by the frequency selectivity of a channel, and without raising the noise level. The interference canceller is fed with decoded symbols (firm decisions) previously produced by a receiver constituted by a simple transversal filter. The difference compared with the invention lies in the fact that firstly our equalizer is fed with weighted decisions generated by a channel decoder, and secondly that we use a process that is iterative. Naturally, the receivers described in [1] and [2] provide performance that is not as good as that of the proposed invention.
Proposals have also been made in:
[3] V. M. Eyuboglu, xe2x80x9cDetection of coded modulation signals on linear, severely distorted channels using decision feedback noise prediction with interleavingxe2x80x9d, IEEE Transactions on Communications, Vol. 36, No. 4, pp. 401-409, April 1988, to use a decision feedback equalizer and a channel decoder combined with a periodic interleaver. The author shows that under some conditions this approach enables the equalizer to make use of the outputs from the decoder by astute management of the delays introduced by the decoder. Naturally, the performance obtained is never better than that of a decision feedback equalizer controlled by the transmitted data. The system described in [3] presents performance that is less good than the proposed invention.
Reference can also be made to:
[4] K. Zhou, J. G. Proakis, and F. Ling, xe2x80x9cDecision feedback equalization of fading dispersive channels with trellis-coded modulationxe2x80x9d, Int. Conf. Commun. Tech., Nanjing, China, November 1987.
[5] K. Zhou, and J. G. Proakis, xe2x80x9cCoded reduced-bandwidth QAM with decision feedback equalizationxe2x80x9d, Conf. Rec. IEEE Int. Conf. Commun., Philadelphia, Pa., pp. 12.6.1-12.6.5, June 1988.
Those two contributions propose a procedure analogous to that followed by V. M. Eyuboglu, but for channels with fading and for a decision feedback equalizer using a criterion of the recursive least-squares (RLS) lattice type. The convergence rate of the equalizer is better than in reference [3], but performance in terms of error rate remains unchanged.
In addition, in:
[6] C. Douillard, A. Glavieux, M. Jxc3xa9zxc3xa9quel, and C. Berrou, xe2x80x9cDispositif de rxc3xa9ception de signaux numxc3xa9riques xc3xa1 structure itxc3xa9rative, module et procxc3xa9dxc3xa9 correspondentsxe2x80x9d [Digital signal receiver apparatus of iterative structure, and corresponding module and method], French patent No. 95 01603, France Txc3xa9lxc3xa9com, TDF, February 1995, it has been proposed to associate a maximum likelihood detector (Viterbi equalizer) with a channel decoder, via an iterative process which makes it possible to manage the various delays introduced by the decoder.
Nevertheless, the apparatus described in that publication is suitable only for channels having response durations that are of the order of a few symbols.
The apparatus proposed by the invention is more particularly adapted to equalizing channels having time responses that are spread over a large number of symbols.
The invention provides equalizing and decoding apparatus including a module which comprises an equalizer and a weighted output decoder, the apparatus being characterized in that it comprises a plurality of modules of said type in series, in which each equalizer has at least one transversal filter for reducing noise power, and in which each module of rank greater than 1 also receives a stream of channel samples that has been delayed by a quantity equal to the processing time of the preceding modules.
Preferably, in each module of rank greater than 1, each equalizer includes another transversal filter which is designed to reconstruct the intersymbol interference present at the output of the first filter and which receives the output from the preceding module, the output of the equalizer corresponding to the output of the first filter minus the intersymbol interference as reconstructed by the second filter.
Advantageously, for modules of rank greater than 1, the transversal filter which receives the stream of channel samples is a filter which converges towards an adapted filter.
Thus, the apparatus comprises a plurality of modules in cascade, each module having an equalizer and a weighted-output decoder.
The equalizer of the module of rank 1 has a transversal filter for reducing both noise power and intersymbol interference. It receives a stream of samples from the demodulator.
In a preferred variant, the equalizer of each module of rank greater than 1 has two transversal filters. The first is adapted to the channel and serves solely to reduce noise power while the second is for reconstructing the intersymbol interference present at the output of the adapted filter. The adapted filter receives a stream of samples from the demodulator delayed by a quantity equal to the processing time of the preceding modules, and the second transversal filter is fed with the stream of symbol mean values as calculated from the output of the channel decoder of the preceding module. The output from the equalizer is equal to the output from the adapted filter minus the intersymbol interference as reconstituted by the second filter.
Thus, with such an iterative structure, the equalizer makes use both of the output from the demodulator and of the output from a channel decoder in an attempt to overcome the intersymbol interference introduced by the selectivity of the channel. It takes account simultaneously of the characteristics of the transmission channel and of the redundancy introduced by the encoding function in order to perform its processing.
It is then possible to obtain performance analogous to that of a channel having additive white Gaussian noise without intersymbol interference and with encoding.
In a variant, the equalizer could equally be of the feedback decision type.